1. Field of the Invention
The present invention relates to a liquid discharger that discharges a droplet of pressurized liquid from an outlet onto an object by pressurizing the liquid with a force generated by a pressure-generating element and a method for adjusting the discharge.
2. Description of the Related Art
As a liquid discharger, an inkjet printer for recording images and text is known. An inkjet printer is advantageous in that the operational cost is low, the size of the apparatus is small, and producing a colored image is easy. The ink of an inkjet printer is stored in an ink cartridge for each color, such as yellow, magenta, cyan, and black, and is supplied to ink chambers in a printer head.
In such an inkjet printer, the ink supplied to the ink chambers is pressurized by a pressure-generating element, such as a heating resistor, disposed inside the ink chamber and, then, is discharged from a minute ink outlet on each ink chamber (i.e., a nozzle). More specifically, the ink in an ink chamber is heated by a heating resistor, and an air bubble is generated inside the ink chamber filled with ink. The size of the air bubble increases and the ink is pressurized until the ink is finally discharged from the nozzle. Images and text are printed by making the discharged ink land on an object such as a sheet of recording paper.
There are two types of inkjet printers: a so-called serial printer and a so-called line printer. For the serial printer, an ink head moves in the width direction of the recording paper (i.e., the direction substantially orthogonal to the feeding direction of the recording paper) to discharge ink of a predetermined color onto a sheet of recording paper. For the line printer, ink is discharged from nozzles aligned along substantially the entire width of the recording paper.
A serial printer stops feeding the recording paper when the ink head moves in the direction substantially orthogonal to the feeding direction of the recording paper. Then, the serial printer prints on the recording paper by repeatedly moving the ink head while ink is discharged on the recording paper.
A line printer generally has a fixed ink head. The line printer prints on a sheet of recording paper being uninterruptedly fed by discharging ink from a linear ink head fixed across the width of the sheet of recording paper.
Accordingly, since the line printer, unlike the serial printer, does not move the ink head, it is advantageous in three points: 1) high printing speed faster than a serial printer is possible; 2) ink capacity can be increased by increasing the size of each ink cartridge; and 3) the structures of head chips, head cartridges, and ink tanks can be simplified.
In the above-described line printer, the recording paper must be fed. Therefore, the printing accuracy of the image and text depends on the accuracy of the timing the ink lands on the recording paper being fed.
To solve such a problem, the timing of the ink landing on the recording paper is controlled in the line printer, for example, by using a servo motor for controlling the feeding speed of the recording paper so that the recording paper is fed at a constant speed and by generating a pulse synchronized with the feeding of the recording paper by an encoder.
Even when a servo motor is used, as described above, expansion and contraction of an image may be prevented, but slight unevenness in the color tone (i.e., unevenness in the density of the color) caused by an instantaneous change in the timing of the ink landing on the recording paper can not be prevented. In other words, if the control of the feeding speed of the recording paper by the servo motor is delayed or quickened instantaneously for merely several microseconds, the landing position of the ink discharged onto this portion of the recording paper will be displaced. Consequently, when a series of ink droplets are discharged, the ink droplets will land close together in some parts and far apart in other parts, causing change in the concentration of the color that appears as unevenness in the density of the color or white stripes. Uneven color density and white stripes appearing in a direction orthogonal to the feeding direction of the recording paper become prominent, for example, when an image is printed at a constant color tone.
In generally, the line printer prints by feeding a sheet of recording paper so that the sheet passes right under the fixed ink head having nozzles aligned in the direction perpendicular to the feeding direction of the recording paper. For this reason, if the discharge direction of the ink discharged from each of the nozzles on the line is not stabilized, a faulty nozzle having a discharge direction different from the other normal nozzles will cause uneven color density and stripes.
On the other hand, for a serial printer, an image can be printed by overlapping the ink. More specifically, by setting a predetermined area where a first printed image and a second printed image overlaps for when an image is printed while the paper feeding is stopped, the concentration (tone) of the color is averaged and unevenness color density and white stripes formed in the feeding direction of the recording paper can be suppressed. Overlapping the ink, however, may prevent uneven color density and white stripes, but, at the same time, may increase the printing time and the amount of ink used for printing.
To solve such problems, a method for controlling the direction of ink discharged from a printer head is disclosed, for example, in Japanese Unexamined Patent Application Publication No. 2000-185403. The discharge direction is controlled by disposing a plurality of heating resistors so that they oppose the nozzles for discharging ink and are plane symmetrical to each other in respect of the plane including the center line of the nozzles to change the heating value of each of the heating resistors.
A head chip having the above-mentioned heating resistors controls the direction of ink discharged from nozzles by changing the heating value of each heating resistor. Therefore, if the heating value of each heating resistor is not controlled appropriately and ink is not discharged in a predetermined direction, the ink does not land on the target landing position on the recording paper. Accordingly, the printed image cannot be improved and degradation of the image cannot be prevented. The landing position is also affected by the distance between the nozzle and the recording paper. When this distance changes, the landing point of the ink droplet also changes, making it difficult to improve the printed image and to prevent degradation of the printed image. To make the ink discharged from each nozzle land at a target landing position, the heating value of each heating resistor (i.e., the amount of energy, such as an electrical current, supplied to each heating resistor for heating each heating resistor) must be determined to obtain a predetermined discharge angle corresponding to the distance between the nozzle and the recording paper.
To determine the amount of energy, such as an electrical current, supplied to one of the heating resistors for heating the heating resistor, the relationship between the discharge angle and the amount of energy must be calculated based on an observation of the trajectory of the ink discharged from the corresponding nozzle, and the distance between the nozzle and the recording paper must be measured. Another method for determining the heating value of each heating resistor for discharging ink at a predetermined discharge angle is to observe the change in the landing positions on the recording paper of the ink discharged at different discharge angles. In this method, however, much equipment including measuring instruments and time are required to calculate the heating value of each heating resistor for discharging ink at a predetermined discharge angle. The structure of the system also becomes large, and reducing the size, weight, and energy consumption becomes difficult.